Make sure you've got the right code.
Let's say, if you go into your Terrains folder, and the .png image for your grayscale heightmap (that's the official name for your map design) is named 'Albinocarrot'.
So, go into Map Maker, and in the terrain box you should type in specifically Albinocarrot.png
You have to have the entire file name in there, including the '.png', since that's how the game renders it.
This should work for you. c: